I'm surprised to read about people in the USA having trouble ordering from Amazon in the UK, or Germany, etc. I have a single Amazon.com (USA) account and I order successfully from their US, UK, German, Canadian stores, (though I've never ordered an ebook from them.) I use the same email address and Amazon password on all of their sites, and all my info (cc, billing and shipping addresses, etc.) pulls up the same across their sites.

It's different for eBooks than for physical goods.
You have to link your entire Kindle library to a single country. Publishing restrictions and such. Lots of books of course are available globally. But some eBooks are limited to certain regions. If you switch to that respective region, you give up your former region.
This could mean, you see all your US eBooks in your then US account, but not the German ones. So you have to link the former German account to the US region, so you see all books.
A bit of a hassle, but easy enough.
Still: I really hope for the "global village"...

Thanks for pointing that out, mg. Since I do not own a Kindle, I never really thought about Amazon (and others, I guess) putting country restrictions on an ebook they sell. Bummer.

It's not so much Amazon, or others, putting restrictions on things as it is publishers only having rights for certain regions due to authors/agents trying to maximize the potential for their pbooks being available in various markets.